ACP Atmospheric Chemistry and Physics ACP 1680-7324 Copernicus GmbH Göttingen, Germany 10.5194/acp-10-8151-2010 Enhancement of the aerosol direct radiative effect by semi-volatile aerosol components: airborne measurements in North-Western Europe Morgan W. T. 1 Allan J. D. 1 2 Bower K. N. 1 Esselborn M. 3 9 Harris B. 4 Henzing J. S. 5 Highwood E. J. 4 Kiendler-Scharr A. 6 McMeeking G. R. 1 Mensah A. A. 6 Northway M. J. 4 Osborne S. 7 10 Williams P. I. 1 2 Krejci R. 8 Coe H. 1 Centre for Atmospheric Science, University of Manchester, Manchester, UK National Centre for Atmospheric Science, University of Manchester, Manchester, UK Institut für Physik der Atmosphäre, Deutsches Zentrum für Luft-und Raumfahrt (DLR), Oberpfaffenhofen, Germany Department of Meteorology, University of Reading, UK TNO, Utrecht, The Netherlands Institute for Chemistry and Dynamics of the Geosphere, Institute 2: Troposphere, Research Centre Jülich, Jülich, Germany Met Office, Exeter, UK Department of Applied Environmental Science, Atmospheric Science Unit, Stockholm University, Sweden now at: European Southern Observatory (ESO), 85748 Garching bei München, Germany now at: Met Office, Cardington, UK 01 09 2010 10 17 8151 8171 This is an open-access article ditributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. This article is available from http://www.atmos-chem-phys.net/10/8151/2010/acp-10-8151-2010.html The full text article is available as a PDF file from http://www.atmos-chem-phys.net/10/8151/2010/acp-10-8151-2010.pdf

A case study of atmospheric aerosol measurements exploring the impact of the vertical distribution of aerosol chemical composition upon the radiative budget in North-Western Europe is presented. Sub-micron aerosol chemical composition was measured by an Aerodyne Aerosol Mass Spectrometer (AMS) on both an airborne platform and a ground-based site at Cabauw in the Netherlands. The examined period in May 2008 was characterised by enhanced pollution loadings in North-Western Europe and was dominated by ammonium nitrate and Organic Matter (OM). Both ammonium nitrate and OM were observed to increase with altitude in the atmospheric boundary layer. This is primarily attributed to partitioning of semi-volatile gas phase species to the particle phase at reduced temperature and enhanced relative humidity. Increased ammonium nitrate concentrations in particular were found to strongly increase the ambient scattering potential of the aerosol burden, which was a consequence of the large amount of associated water as well as the enhanced mass. During particularly polluted conditions, increases in aerosol optical depth of 50–100% were estimated to occur due to the observed increase in secondary aerosol mass and associated water uptake. Furthermore, the single scattering albedo was also shown to increase with height in the boundary layer. These enhancements combined to increase the negative direct aerosol radiative forcing by close to a factor of two at the median percentile level. Such increases have major ramifications for regional climate predictions as semi-volatile components are often not included in aerosol models. <br><br> The results presented here provide an ideal opportunity to test regional and global representations of both the aerosol vertical distribution and subsequent impacts in North-Western Europe. North-Western Europe can be viewed as an analogue for the possible future air quality over other polluted regions of the Northern Hemisphere, where substantial reductions in sulphur dioxide emissions have yet to occur. Anticipated reductions in sulphur dioxide in polluted regions will result in an increase in the availability of ammonia to form ammonium nitrate as opposed to ammonium sulphate. This will be most important where intensive agricultural practises occur. Our observations over North-Western Europe, a region where sulphur dioxide emissions have already been reduced, indicate that failure to include the semi-volatile behaviour of ammonium nitrate will result in significant errors in predicted aerosol direct radiative forcing. Such errors will be particularly significant on regional scales.

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